Assuming your tank is around 5' by 2', and you wanted 3" substrate depth, you'd need 240lbs of Eco Complete, which is 12 bags. Eco Complete is inert, but is good at absorbing nutrients for future use. As specific aquarium-use substrates go, Eco Complete is about as cheap as you'll find (unless you go with clown puke gravel), but your best bet would honestly be to make some MTS or use Miracle Gro Organic Potting Mix and cap it with pool filter sand. This would cost you $25-35, rather than the $200-250 it would cost for the Eco.
...unless money isn't an issue at all, in which case, skip the Eco and pick something off the ADA website.